In semiconductor device manufacturing, photolithography is used in printing features of semiconductor devices onto a wafer whereupon the devices are to be formed. Quality of the patterns or features of the devices printed by the photolithography is generally measured by resolution and particularly by the size of a resolvable half-pitch (Pmin/2). It is well known in the art that the minimum resolvable half-pitch that a photolithographic exposure system may deliver is determined by the Rayleigh criterion, Pmin/2=k1·λ/NA, where λ is the wavelength of light used in the exposing process, NA is the numerical aperture of the objective lens of the system, and k1 is a process-dependent factor. For decades, resolution of photolithographic exposure systems has steadily improved because of, even though incremental, decreases in λ, increases in NA, and decreases in k1. For example, historically, the minimum resolvable half-pitch is scaled down by about 30% every two years.
However, single exposure photolithography, which has been the mainstream approach of semiconductor industry, is quickly reaching its physical barrier for continuing to be applied to devices of ever shrinking in size. For example, printing a 22 nm node (with a 32 nm half-pitch) at an NA of 1.35 is becoming almost impossible with a single exposure system simply because the k1 factor will have to drop to around 0.22, which is below the theoretical limit of 0.25 as is known in the art. Unlike all previous generations of photolithography, no next-generation exposure tools with higher NA are expected to be ready or available in time to enable single exposure for 22 nm node production.
As an alternative to single exposure photolithography, double-patterning technology (DPT) is now emerging as a major candidate for printing 22 nm node optically and as a way for easing requirements on printing current 32 nm node. Double patterning technology enables pitches being printed with resolutions smaller than the minimum resolvable half-pitch that a single exposure system can deliver. Even though the theoretical 0.25 limit of k1 factor will still hold, the resulting patterns on the wafer will appear as if a lower k1 has been used. For example, when features at a 60 nm pitch are printed as two interleaved 120 nm pitches, using the so-called “pitch split” approach, even though the actual k1 is 0.42, an effective k1 of 0.21 may be achieved with an objective lens of NA at 1.35.
Double patterning technology may also be used in printing types of features that are at odds with one another in a single exposure process. These types of features are becoming increasingly common as increases in NA—while boosting the depth of focus (DOF) of dense pitches—start to erode the depth of focus of some isolated pitches.
For example, in single exposure schemes, sub-resolution assist features (SRAFs) are usually placed in proximity to isolated features to make them appear denser and to increase their depth of focus. SRAFs are traditionally placed in a layout arranged according to a table of rules that includes, among other things, distance from main features, number of assists to place, and width of the assists. Although SRAFs have historically been a useful tool for increasing process windows of isolated features they are increasingly becoming difficult to be implemented. This is because width of assist features needs to be kept significantly below a minimum feature critical dimension (CD) such that the assist features do not print, hopefully across all possible doses and focus range. As feature CDs continue to shrink, size of SRAFs shrinks as well which makes mask manufacturing and inspection more difficult. Process window may be sufficiently increased only if the SRAF spacing and/or width of the SRAF are set such that printing of assist features occurs. In a single exposure process, printing of these assist features can lead to defects in the patterned structure.
To help mitigate above difficulties relating to SRAF, a double patterning technique, in particular a complementary double exposure (CODE) technique, was first proposed for the 90 nm node. The technique was a double-expose-double-etch (DE2) process during which large extraneous features were printed in a first exposure and then removed in a second exposure. When being printed in the first exposure, the extraneous features were placed adjacent to critical features to improve process windows of the critical features. A variety of themes were explored, but in general the focus of the technique was on improving depth of focus (DOF) of the isolated features by introducing extraneous features that tend to increase the pattern density.
The CODE process in general enhances or improves the effectiveness of extraneous features by allowing these extraneous features to print. As described above, the extraneous features are printed in a first exposure and subsequently removed in a second exposure. Since these extraneous features are no longer “sub-resolution” due to the nature of them being printed, they are generally referred to as printing assist features (PrAFs) in the industry. In a standard optical proximity correction (OPC) process flow, the PrAFs are generally treated like regular features, and receive their respective corrections which may be deemed as appropriate by the OPC process.
In a double exposure scheme, appropriate process and/or methodology are needed in order to separate a design layout into two exposure steps. For example in a CODE scheme, a process, or method, may be developed to add certain PrAF shapes to a mask in one exposure, and to generate trim shapes that will remove the PrAF shapes in a follow-up exposure. This process is generally known as a data decomposition process or step in the overall lithographic process. Generally, a design layout, as being received from a designer, contains no explicit information about how it may be decomposed. Therefore, the data decomposition process, which is normally implemented in data preparation software, needs to create mask layouts for the two exposures. Data decomposition may be trivial for layouts that are one-dimensional (1D), however proper separation or decomposition of random two-dimensional (2D) layouts is generally considered exceedingly difficult, if not impossible. In order to ensure that design layouts are decomposed properly, proper RET and design rules need to be established to prevent any unworkable cases.
Establishment of proper rules for PrAF placement is complicated. It is significantly more complicated than, for example, a SRAF process because a layout designer has many more options to choose from in deciding a proper PrAF placement. For example, rather than being restricted to a limited number of different widths like in a SRAF scheme, PrAFs may be designed with a variety of choices of widths and placement styles that can be adjusted and/or optimized according to, for example, pitches and feature types. A general methodology and/or approach for finding a proper solution, and in some instances optimized solution, from various PrAF options is needed for the lithographic industry relating to recent resolution enhancement technology (RET).
So far, there is no known art that addresses how to place and adjust PrAFs in a double patterning process and addresses how to fit the adjustment of PrAFs into a general data decomposition process. In other words, there is no known art that would enable the use of PrAFs such that they may be applied to layouts using data preparation software.